Method of forming lens surfaces



p 1945- Q l H. PENBERTHY 2,384,638

METHOD OF FORMING LENS SURFACE Filed 061;. 4, 1943 FIGJ.

3 FIG. 2

FIG. 3.

HARVEY LARRY PENBERTHY INVENTOR ATTORNEY Patented Sept. 11, 1945 METHODOF FORMING LENS SURFACES Harvey Larry Penberthy, Rochester, N. Y., as-

signor to, Eastman Kodak Company, Rochester, N. Y., a corporation of NewJersey Application October 4, 1943, Serial No. 504,897

2 Claims.

This invention relates to the shaping of lens surfaces. Such surfacesare ordinarily formed by molding or grinding, the most common methodbeing molding to a very rough approximation of the desired shape andthen grinding, followed by polishing of the surface, the polishingusually being with pitch and rouge.

In some cases ac? d polishing is used to attain the final surfaceefiect, this comprising treatment with hydrofluoric acid. Acidpolishing, however, is used merely to attain the desired finish, and thehydrofluoric acid treatment has not been used to shape the lens surfaceto the desred curvature. In the patent Correll, 1,430,494, grantedSeptember 26, 1922, it is stated that hydrofluoric acid may be used togive a satin-like sheen and that it may be used in conjunction withcarborundum or other fine powder to enhance the effect. The acid is, inno instance, however, used as the primary means for attacking orremoving the substance of the glass under conditions so controlled as toshape the surface to the desired curvature.

I have found that if the hydrofluoric acid is applied to the surface ofthe glass blank in controlled amount, and at the same time the surfaceis rubbed with a non-abrasive curve generator which merely removes theproducts of the acid treatment, the glass will assume the same sphericalshape that it would if no acid were used and the curve generator had theusual abrasive surface. The action of the curve generator is to removethe products of the acid attack from the high points, leaving them opento further attack, while the unremoved products at low points in thesurface partially protect these points from fur ther attack. The actionthus proceeds until the whole surface has assumed the spherical form ofthe curvature for which the curve generator is set.

For cleamess of description reference will now be made to theaccompanying drawing wherein:

Fig. 1 shows diagrammatically a curve generator used with a singleconvex lens blank;

Fig. 2 shows similarly a curve generator used with a blocking bodyholding a multiplicity of convex lenses;

Fig. 3 shows similarly a curve generator used with a single concavelens, it being understood that the blocking body may hold a multiplicityof concave lenses.

In Fig. 1 a lens blank I, which in this instance has previously beenroughly molded with one surface convex, is mounted on a support 2carried by the rotatable shaft 3. A water-tight basin 1 surrounds thesupport 2 and is rotatable with it. A cup-shaped element 4, shown insection, having a rim 6 similar in form to known curve generators inwhich the face of the rim is of abrasive material is carried byrotatable shaft 5, the angle of the latter being adjustable as in knownlens grinding machines in which the curvature of the round surface isdetermined chiefly by the angular position of the shaft 5. In thepresent case, however, the rim is of a material which is not abrasive toglass in the sense that diamond and corundum are and yet is hard enoughto remove the products of the acid attack. Plastics of known celluloseacetate compositions, methacrylate resins, ureaformaldehyde condensationproducts, pitch, hard rubber, lead, copper, brass, Monel metal andanalogous known materials may be used. These materials may or may not becorrodible by hydrofluoric acid. If the material is slightly corrodibleby the acid, a smoother lens surface is obtained; if not corrodible, thecup has a longer life.

In operation, a lens blank is fastened in place on the lower shaft 3,and hydrofluoric acid, HF, is

'poured into the basin 1. The two rotatable shafts,

3 and 5, are then set in motion, being rotated in opposite directions,and one of them, usually 3, is advanced along its axis until therevolving cup contacts the revolving lens. The contact is maintained bymoderate pressure until all of the high spots of the lens are corrodedaway, as described above. Most of the acid is then removed, and

water is added while the machine is still operating. The then diluteacid continues to attack the glass, but at a much slower rate, helpingto obtain an even surface. The dilution may be continued until thechemical action ceases. Alternatively, the action can be stopped at onceby the addition of an alkali. The machine is then stopped and the lensremoved. The lens surface thus prepared is sufiiciently smooth andspherical for many applications, such as cementing to another lens oruse as a condensing lens. If high accuracy is desired, a small amount ofadditional polishing by usual optical methods will produce theperfection required. It is to be understood that the acid may be appliedto the surfaces by other means than immersion.

In Fig. 2 the parts are the same as in Fig. 1, where the same referencecharacters are used, but instead of carrying a single lens the support 2carries a blocking body 8 holding a plurality of lens blanks 9, and thecup wheel I 0 has wide rims II.

The cup wheels 4 and III are shown without serv 9 if the width of therim of the wheel is greater,

as in I I of Fig. 2 compared with 6 of Fig. 1. In 1 act, the wheel neednot have any hole in the center at all, although experience shows thatvery little, if anything, is gained thereby.

In Fig. 3, also, like characters designate like parts, but in this casethe single lens l2 shown is concave, and the position and shape of thecup wheel I! are adapted to form the desired concave surface. It isunderstood that a concave blocking body carrying several lens blankscould also be used.

As described above, the acid attacks the glass with the formation ofproducts which, adhering to the glass, inhibit the further attack of theacid unless they are removed, as by the action of the cup wheel uponhigh spots on the lens blank. This inhibiting action can be extended tothe point ofbeing an actual resist by coating the lens,

before surfacing, with wax, lacquer, or a similar substance whichprevents action of the acid but which will beremoved locally by therotating cup wheel when the wheel comes in contact with said substance.

The foregoing describes only the use of hydrofluoric acid and water, andin general this is suflicient. However, the fluorides of some of theelements used in certain glasses are very hard to remove. In such casesit is better to add other chemical reagents to the hydrofluoric acid tochange the products of the attack of the glass and hence their physicalproperties. Many chemical reagents, such as nitric acid, sulfuric acid,phosphoric acid, hydrochloric acid, perchloric acid, acetic acid,potassium dichromate, sodium chloride, sodium nitrate, and so forth, canbe used to accomplish the same result. Further, plain nitric acid orsulfuric acid alone works very well 45 asseesa with certain glasses suchas very dense barium glasses having a low silicate content and thenonsilicate glasses described in the patents, Eberlin 2,206,081 andEberlin and De Paolis 2,241,249. These few instances by no means exhaustthe possibilities of making changes in the chemical constitution of thecorrosive bath and should not be construed as limitations of the methoditself.

Having thus described my invention, what I claim is:

1. The method of forming a lens surface of desired curvature from arough glass blank that comprises coating the surface with a resist,applying to the surface of the blank a chemical which attacks glass butdoes not penetrate the resist, rotating the blank in contact with arotating non-abrasive surface having a position and shape conforming, tothe desired lens surface, whereby the non-abrasive surface wears awaythe resist from protuberances in the glass surface, thus permittlngaccess of the chemical to the glass, and also removes the products ofthe action of the chemical upon the glass, and continuing theapplication of chemical and rotating of the said surfaces in contactuntil the resist is entirely removed and the glass surface has attainedthe desired curvature.

2. The method. of forming a, lens surface of desired curvature from arough glass blank that comprises coating the surface with a materialthat is resistant to hydrofluoric acid, applying hydrofluoric acid tothe coated surface and at the same time rotating the blank in contactwith a rotating non-abrasive surface having a position and shapeconforming to the desired curvature,

whereby the non-abrasive surface first wears awaythe resist fromprotuberances in the glass surface, thus permitting access of the acidto the glass, and also removes, as it forms, the products of the actionof the acid upon the glass, and continuing the application ofhydrofluoric acid and the rotation of the surfaces in contact until theresist is entirely removed and the glass surface has attained thedesired curvature.

HARVEY LARRY PENBERTHY.

